add increased residual gas damping to due squeezed film damping

gwinc/ifo/aLIGO/ifo.yaml

@@ -50,6 +50,14 @@ Infrastructure:
       mass: 2.99e-26
       polarizability: 1.71e-30
 
+    # Excess gas damping due to compensation plates and reaction masses
+    # Numbers are rough guesses from LIGO-T0900582
+    # ExcessDamping is \sqrt{\Delta S_{F_0}} of (5) of
+    # http://dx.doi.org/10.1103/PhysRevD.84.063007
+    ExcessDamping: 2.6e-15  # N/rtHz; excess force noise; section 6
+    DiffusionTime: 800e-6   # s; diffusion time; section 4
+
+
 TCS:
   # The presumably dominant effect of a thermal lens in the ITMs is an increased
   # mode mismatch into the SRC, and thus an increased effective loss of the SRC.

gwinc/noise/residualgas.py

@@ -86,7 +86,41 @@ def residual_gas_damping(f, ifo, species, sustf):
     beta_inf = pi * radius**2 * pressure/thermal_vel
     beta_inf *= (1 + thickness/(2*radius) + pi/4)
 
+    force_noise = 4 * kT * beta_inf
+
+    # add squeezed film damping if necessary as parametrized by (5)
+    if 'ExcessDamping' in ifo.Infrastructure.ResidualGas:
+        # the excess force noise and diffusion time are specified directly
+        # FIXME: probably need to scale this by the partial pressure and
+        # thermal velocity of each species to paremetrize it correctly
+        DeltaS0 = ifo.Infrastructure.ResidualGas.ExcessDamping**2
+        try:
+            diffusion_time = ifo.Infrastructure.ResidualGas.DiffusionTime
+        except AttributeError:
+            msg = 'If excess residual gas damping is given a diffusion time ' \
+                + 'must be specified as well'
+            raise ValueError(msg)
+
+    elif 'gap' in ifo.Infrastructure.ResidualGas:
+        # if a gap between the test mass and another object is specified
+        # use the approximate model of section IIIA and B
+        gap = ifo.Infrastructure.ResidualGas.gap
+
+        # Eq (14)
+        diffusion_time = sqrt(pi/2) * radius**2 / (gap * thermal_vel)
+        diffusion_time /= log(1 + (radius/gap)**2)
+
+        # Eq (11) factoring out the low pass cutoff as in (5)
+        DeltaS0 = 4 * kT * pi*radius**2 * pressure * diffusion_time / gap
+
+    else:
+        DeltaS0 = 0
+        diffusion_time = 0
+
+    # Eq (5)
+    force_noise += DeltaS0 / (1 + (2*pi*f*diffusion_time)**2)
+
     # convert force to displacement noise using the suspension susceptibility
-    noise = 4 * kT * beta_inf * abs(sustf.tst_suscept)**2
+    noise = force_noise * abs(sustf.tst_suscept)**2
 
     return noise